The present invention relates to techniques for drilling oil, gas, water, or geothermal wells or the like. More precisely, the invention relates to cementing compositions which are particularly suitable for cementing zones which are subjected to extreme static or dynamic stresses.
In general, a well which is over a few hundred meters deep is cased and the annular space between the underground formation and the casing is cemented over all or part of its depth. Cementing essentially prevents the exchange of fluid between the different layers of formation traversed by the hole and controls the ingress of fluid into the well, and in particular limits the ingress of water. In production zones, the casingxe2x80x94and the cement and the formationxe2x80x94are perforated over a height of several centimeters.
The cement placed in the annular space of an oil well is subjected to a number of stresses throughout the lifetime of the well. The pressure inside a casing can increase or decrease because the fluid which fills it can change or because additional pressure is applied to the well, for example when the drilling fluid is replaced by a completion fluid, or during a stimulation operation. A change in temperature also creates stress in the cement, at least during the transition period before the steel and the cement reach temperature equilibrium. In the majority of the above cases, the stress process is sufficiently slow for it to be treated as a static process; and in some cases the forces can be sufficiently large to damage the casing. The cement is also subjected to other stresses which are dynamic in nature, either because they are produced over a very short period or because they are either periodic or repetitive in nature. Perforating creates an over-pressure of several hundred bars inside a well which is dissipated in the form of a shock wave. Further, perforating creates a shock when the projectile penetrates the cement and that shock subjects the zone surrounding the hole to large forces over a depth of several meters.
A further process, which is now routine in oil well, operations and which creates dynamic stresses in the cement, is the opening of a window in a casing which is already cemented to create a multi-branch lateral well. Milling the steel over a depth of several meters followed by drilling a lateral well subjects the cement to shocks and vibrations which usually damage it irreparably.
The present invention aims to provide novel formulations, in particular for cementing regions of oil wells or the like which are subjected to extreme static or dynamic stresses.
An article presented at the SPE (Society of Petroleum Engineers) annual technical conference and exhibition of 1997, Marc Thiercelin et al. (SPE 38598, 5-8 Oct. 1997)xe2x80x94and French patent application. FR-A 97 11821 of 23rd Sep. 1997xe2x80x94demonstrate that the risk of rupture of a cement sleeve depends on the thermoelastic properties of the casing, of the cement, and of the formation surrounding the well. A detailed analysis of the mechanisms leading to rupture of the cement sleeve has shown that the risk of rupture of a cement sleeve following an increase in pressure and/or temperature in the well is directly linked to the tensile strength of the cement and is attenuated when the ratio of the tensile strength RT of the cement to its Young""s modulus E is increased.
Young""s modulus is known to characterize the flexibility of a material. To increase that RT/E ratio, it is advantageous to select materials with a low Young""s modulus, in other words to select very flexible materials.
One known means for increasing the flexibility of a hardened cement is to reduce the density of the slurry by extending it with water. However, that leads to the stability of the slurry being degraded, in particular with the solid and liquid phases separating. Such phenomena can, of course, be controlled in part by adding materials such as sodium silicate, but the permeability of the hardened cement is nevertheless very high, which means that it cannot fulfill its primary function of isolating zones to prevent fluid migration, or at least it cannot guarantee its long-term isolation. Furthermore, lightened cements have lower strength, in particular lower shock resistance, which constitutes a clear handicap for cements intended for use in zones which are subjected to extreme mechanical stresses such as perforation zones.
In the building field, incorporating particles of rubber into a concrete is known to result in better toughness, durability and elasticity [see, for example, A. B. Sinouci, Rubber-Tire Particles as Concrete Aggregate, Journal of Materials in Civil Engineering, 5, 4, 478-497 (1993)]. Concretes which include rubber particles in their formulation can be used, for example, in highway construction to absorb shocks, in anti-noise walls as a sound insulator, and also in constructing buildings to absorb seismic waves during earthquakes. In such applications, the mechanical properties in particular are improved.
In the field of oil well cementing, it is also known [Well Cementing 1990, E. B. Nelson, Schlumberger Educational Services] that adding ground rubber particles (with a grain size in the range 4-20 mesh) can improve impact strength and bending strength. Such an improvement in mechanical properties has also been indicated in patent publications SU-1384724 and SU-1323699. More recently, United States patent, U.S. Pat. No. 5,779,787 has proposed the use of particles derived from recycled automobile tires in a grain size in the range 10/20 or 20/30 mesh, to improve the mechanical properties of hardened cements, in particular to improve their elasticity and ductility.
In its French patent application number FR-A-98 16104 dated 21st Dec. 1998, the Applicant describes lightened oil cements reinforced with flexible particles, of low compressibility, with low density, and with an average particle size not exceeding 500 micrometers (xcexcm). The density of the flexible particles is less than 1.5 grams per cubic meter (g/cm3), preferably less than 1.2 g/cm3 and more preferably less than 1 g/cm3. That results in lightened cement slurries. All of the examples given in French patent application FR-A-98 16104 correspond to a slurry with a density of less than 1.68 g/cm3.
Further, in patent application FR-A-98 12538, the Applicant also describes cement slurries with a density of less than 1.70 g/cm3 comprising 30% to 100% (by weight of cement) of rubber particles, with a grain size of 40-60 mesh with a diameter which is preferably in the range 250 xcexcm to 400 xcexcm.
As indicated above, a reduction in slurry density tends to encourage its flexibility and is thus usually desired. However, low density may not be desirable in particular when the pressure is high due to the formation or to the nature of other fluids pumped downstream or upstream of the lightened slurry.
The authors of the present invention have set themselves the target of producing cementing slurries reinforced by flexible or rubber particles with a low density, but with the density of the slurries themselves being normal for cementing slurries for oil wells or the like, i.e., typically in the range 1.7 g/cm3 to 2.2 g/cm3.
According to the invention, the problem is solved by cementing slurries for an oil well or the like comprising an hydraulic binder, dense particles with a density higher than the density of the hydraulic binder and reinforcing particles with a density of less than 1.5 g/cm3, preferably less than 1.2 g/cm3, constituted by a rubber or a flexible material, of low compressibility and with an average grain size of less than 600 micrometers (xcexcm).
The flexible particles are constituted by a material with a Young""s modulus of less than 5000 mega Pascals (MPa) (preferably less than 3000 MPa, more preferably less than 2000 MPa), i.e., the elasticity of these particles is at least four times greater than that of cement and more than thirteen times that of the silica usually used as an additive in oil well cements. The flexible particles added to the cementing compositions of the invention are also remarkable because of their low compressibility. Materials which are more compressible than rubbers, in particular with a Poisson ratio of less than 0.45, preferably less than 0.4, are preferred. However, materials which are too compressible, with a Poisson ratio of less than 0.3, are not preferred.
Preferred dense particles are particles with a specific gravity of well over 3, such as haematite particles which have a density of 4.95 g/cm3.
The dense particles and reinforcing particles must be insoluble in an aqueous medium which may be saline, and they must be capable of resisting a hot basic medium since the pH of a cementing slurry is generally close to 13 and the temperature in a well is routinely higher than 100xc2x0 C.
Regarding the size of the rubber or flexible particles, essentially isotropic particles are preferred. Spherical or near spherical particles may be synthesized directly, but usually the particles are obtained by grinding, in particular cryo-grinding. The average particle size is generally in the range 80 xcexcm to 600 xcexcm, preferably in the range 100 xcexcm to 500 xcexcm. Particles which are too fine, also particles which are too coarse, are difficult to incorporate into the mixture or result in pasty slurries which are unsuitable for use in an oil well.
Particular examples of materials which satisfy the various criteria cited above are thermoplastics (polyamide, polypropylene, polyethylene, . . . ) or other polymers such as styrene divinylbenzene or styrene butadiene (SBR).
In addition to flexible particles and dense particles, the cementing compositions of the invention comprise an hydraulic binder, in general based on Portland cement and water. Depending on the specifications regarding the conditions for use, the cementing compositions can also be optimized by adding additives which are common to the majority of cementing compositions, such as suspension agents, dispersing agents, anti-foaming agents, expansion agents (for example magnesium oxide or a mixture of magnesium and calcium oxides), fine particles, fluid loss control agents, gas migration control agents, retarders or setting accelerators.
A typical composition of the invention comprise, by volume, 2% to 15% of dense particles, 5% to 20% of flexible particles, 20% to 45% of cement and 40% to 50% of mixing water.
The formulations of the invention are preferably based on Portland cements in classes A, B, C, G and R as defined in Section 10 of the American Petroleum Institute""s (API) standards. Classes G and H Portland cements are particularly preferred but other cements which are known in this art can also be used to advantage. For low-temperature applications, aluminous cements and Portland/plaster mixtures (for deepwater wells, for example) or cement/silica mixtures (for wells where the temperature exceeds 120xc2x0 C., for example) can be used, or cements obtained by mixing a Portland cement, slurry cements and/or fly ash.
The water used to constitute the slurry is preferably water with a low mineral content such as tap water. Other types of water, such as seawater, can possibly be used but this is generally not preferable.
These particles with low density with respect to the cement can affect the flexibility of the system, since adding flexible particles produces cements with a lower Young""s modulus, while producing low permeability and better impact resistance.
The mechanical properties of the compositions comprising flexible particles of the invention are remarkable, rendering them particularly suitable for cementing in areas of an oil well which are subjected to extreme stresses, such as perforation zones, junctions for branches of a lateral well or plug formation.